Prior art tracking novelty filters have been employed at least since the early days of radar, when they were used to keep radar screens from becoming cluttered by nonmoving objects. A tracking novelty filter is readily implemented using a digital computer to subtract incoming images, pixel by pixel, from a stored reference image that is periodically updated. In 1987, Anderson, Lininger, and Feinberg proposed the concept of constructing a tracking novelty filter using a Michelson interferometer with a phase conjugate mirror. Such optical systems have the advantage of fast operation because of the parallelism of light.
Previous publications and demonstrations of such systems have involved the use of an interferometric configuration including a self-pumped phase conjugate mirror formed of barium titanate. This approach exhibits a slow response time and is therefore only suitable for detecting moving objects in a steady background. This approach is not sufficient for detecting fast (or accelerating) moving objects in a more realistic noise environment cluttered with other time-varying background.